1. The Field of the Invention
The invention relates to a method for generating digital still pictures of wafer-shaped elements in a production process and relates to a device for executing this method. In particular but not exclusively the invention is applicable to processes for producing photovoltaic or solar cells from wafer-shaped elements which are formed as semiconductor slices or wafers.
2. Description of the Related Art
When manufacturing solar cells the single semiconductor slices or wafers are transported on a conveyor belt, inter alia, for the purposes of optical quality control. Usually this optical quality control or inspection is performed with the assistance of digital image data processing. The purpose is to find out if there are defective areas or spots on the solar cell or wafer. Since the wafers are arranged to one another on the conveyor belt, it is necessary to identify each single wafer separately. Usually sensors are used for it, namely so-called initiators, such as light barriers, ultrasonic sensors, capacitive sensors, mechanical or pneumatic sensors, etc.
Since the wafers are transported on the conveyor belt one after another it is necessary to clearly identify each wafer. For this purpose it is known to use at least one sensor or initiator which detects the beginning and the end of each wafer on the basis of the gaps appearing between two sequenced wafers. However it may happen that adjacent wafers are arranged on the conveyor belt very close to each other so that no gap can be detected. Thus the detection by means of initiators or sensors fails so that the succeeding image data processing for the optical quality inspection is hindered. In particular it can become very complicated or even impossible to make still pictures of the wafers or the solar cells. Thus no corresponding digital single pictures can be extracted which are needed decide whether to sort out the products on the basis of their quality characteristics.
DE 103 45 581 A1 discloses a method of detecting objects being transported on a conveyor belt by taking pictures with the help of a line scan camera. The objects are automobile parts, such as car doors, having edges and holes to be detected for mounting and assembly in a car production process.
DE 103 45 581 A1 discloses a method of detecting objects being transported on a conveyor belt by taking pictures with the help of a line scan camera. The objects are automobile parts, such as car doors, having bore-holes to be detected and measures within a car production process. Thus the method performs a partial scan of a small detailed part of each object, such as a specific bore-hole. It is not described to make a full scan of the entire object (e.g. the door) or to detect gaps between the objects which are sequenced on the conveyor belt.
DE 101 40 741 C1 discloses a device and a method being applied to CT equipment (CT: computer tomography) to improve the positioning of image data generated by a rotating data acquisition system (DAS). The DAS is triggered by a trigger signal in dependence of the angular position of the sensor. If the system fails to generate a trigger within a preset time interval a synthetic trigger is generated. The described solution is thus directed to the synchronization by trigger signals, but it does not describe how to generate digital still pictures of elements or objects transported on a conveyor belt within a production process.